•
Hydrogenation
Addition reaction process by which alkenes are reduced to alkanes
•
Reduction
Increases electron density on carbon by
•
• Forming CH
Breaking CO , CN , or CX bond
Reduction of Alkenes: Hydrogenation
•
•
•
Catalytic hydrogenation
A heterogeneous process that takes place on the surface of insoluble catalyst particles
Common catalysts for alkene hydrogenation:
•
• Platinum
– PtO
2
(
Adams’ Catalyst
)
Palladium
– very fine powder supported on inert material such as charcoal (Pd/C)
Occurs with syn stereochemistry
• Both hydrogens add to the double bond from the same side
Reduction of Alkenes: Hydrogenation
Mechanism of catalytic hydrogenation
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Reduction of Alkenes: Hydrogenation
Other unsaturated functional groups are much less reactive toward catalytic hydrogenation under normal reaction conditions.
Reduction of Alkenes: Hydrogenation
•
Hydrogenation
Unsaturated vegetable oils reduced to produce saturated fats used in margarine and cooking products
• Vegetable oils
•
Triesters of glycerol, HOCH
2
CH(OH)CH
2
OH, with three long-chain carboxylic acids called fatty acids
•
Fatty acids
• Polyunsaturated carboxylic acids containing long hydrocarbon chains
• Double bonds have cis stereochemistry
Reduction of Alkenes: Hydrogenation
Catalytic hydrogenation of polyunsaturated fats
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Reduction of Alkenes: Hydrogenation
Catalytic hydrogenation of polyunsaturated fats
•
•
Complete hydrogenation leads to saturated fatty acids
Incomplete hydrogenation results in isomerized trans fats that release trans fatty acids upon digestion, increasing blood cholesterol levels
Biological hydrogenation (reduction) of isolated double bonds
•
•
•
Double bond must be adjacent to a carbonyl group
The reduction of isolated double bonds is rare in biological pathways
Process occurs in two steps
1.
NADPH (coenzyme reduced nicotinamide adenine dinucleotide phosphate) adds hydride ion (H : ) to double bond to produce an ion
2.
Protonation of an anion by acid HA leading to an overall addition of H
2
Reduction of Alkenes: Hydrogenation
Biological reduction of double bond in trans -Crotonyl
ACP leads to the formation of Butyryl ACP
8.6 Oxidation of Alkenes: Epoxidation
•
Oxidation
A reaction that results in a loss of electron density by carbon
•
Oxidation
Decreases electron density on carbon by
• Breaking CH bond
• Forming CO , CN , or CX bond
Note: oxidation often adds oxygen; reduction often adds hydrogen
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Oxidation of Alkenes: Epoxidation
Alkenes on treatment with a peroxyacid, RCO
3
H, are oxidized to give epoxides
•
Epoxide (oxiranes)
Cyclic ethers with an oxygen atom in a three-membered ring
Oxidation of Alkenes: Epoxidation
•
•
•
•
Synthesis of epoxides from alkenes
Peroxyacid transfers oxygen to alkene
Syn stereochemistry
• Both C-O bonds form on the same face of the double
One step mechanism
No intermediates
Oxidation of Alkenes: Epoxidation
•
•
•
•
Synthesis of epoxides from halohydrins
Preparation of halohydrin through electrophilic addition of
HO-X to alkene
Treatment of halohydrin with base deprotonates OH
O
nucleophile reacts with CCl electrophile substituting C-
O bond for CCl bond
Cl
eliminated yielding the epoxide
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Oxidation of Alkenes: Epoxidation
•
•
•
Epoxides in biological pathways:
Epoxides prepared from alkenes as intermediates
Peroxyacids are not involved
• FADH
2 used in biological reactions
Conversion of squalene into 2,3- oxidosqualene; a key step in the biosynthesis of steroids
•
•
Hydroxylation
The addition of an –OH group to each of the two double-bond carbons
Two step process:
1.
2.
•
Epoxidation
Hydration
Epoxides undergo an acid-catalyzed reaction with water to give corresponding 1,2-dialcohol, or diol
Oxidation of Alkenes: Hydroxylation
Acid catalyzed epoxide-opening takes place by:
1.
2.
Protonation of the epoxide increasing the electrophilicity of carbon
•
Nucleophilic addition of water followed by deprotonation
Trans -1,2-diol formed
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Oxidation of Alkenes: Hydroxylation
•
•
•
Hydroxylation in the Laboratory
•
•
Carried out directly by oxidation of an alkene with osmium tetroxide, OsO
4
Catalytic amount of OsO
4 used in the presence of stoichiometric amount of N -methylmorpholine N -oxide (NMO)
Syn stereochemistry
No carbocation intermediate
Occurs through cyclic osmate intermediate
8.8 Oxidation of Alkenes: Cleavage to
Carbonyl Compounds
•
•
•
Ozone (O
3
) is useful double-bond cleavage reagent
Ozone is generated by passing a stream of oxygen through a highvoltage electrical discharge
Ozone adds rapidly to C=C bond at low temperature to give molozonide which spontaneously rearranges to ozonide
Ozonide is treated with reducing agent to convert it to carbonyl compounds
Oxidation of Alkenes: Cleavage to Carbonyl
Compounds
•
•
If tetrasubstituted double bond is ozonized, two ketone fragments result
If a carbon of the alkene is bonded to hydrogen, ozonolysis will cleave the double bond to yield an aldehyde
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Oxidation of Alkenes: Cleavage to Carbonyl
Compounds
•
•
Potassium permanganate (KMnO
4
) in neutral or acidic solution cleaves alkenes to give carbonyl-containing products
If a carbon of the alkene is bonded to hydrogen a carboxylic acid is produced
If a carbon of the alkene is bonded to two hydrogens, CO
2 is formed
Oxidation of Alkenes: Cleavage to Carbonyl
Compounds
•
•
Alkenes are also cleaved by hydroxylation to a 1,2-diol followed by treatment with periodic acid, HIO
4
.
If the two
–OH groups of the diol are in an open chain, two carbonyl compounds result
If the two –OH groups of the diol are on a ring, a single, open-chain dicarbonyl compound is formed
Worked Example 8.3
Predicting the Reactant in an Ozonolysis
Reaction
What alkene would yield a mixture of cyclopentanone and propanal on treatment with ozone followed by reduction with zinc?
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